1. Field of the Invention
The present invention relates generally to supplying power for electric systems. More particularly, the invention relates to systems and methods for managing the energy supplied from a power source to accommodate peak power requirements of a load.
2. State of the Art
The power density of an all electric system is often a limiting factor which restricts design flexibility. For example, an electric system requiring 50 kilowatts for 1 second, but only requiring 1 kilowatt during all other normal operations can be considered to have an average power of slightly over 1 kilowatt, but a peak power of 50 kilowatts. In this electric system, the 1 kilowatt operating state represents a low power state, while the 50 kilowatt peak power represents a short-term, high power state. Such a system is designed with electrical components that are compatible with peak power requirements of the high power state.
Exemplary systems wherein at least two such states exist include electric motor control systems. In such a system, a high power state can exist during periods of motor acceleration (e.g., motor start-up). Other exemplary systems where dual states of a power supply are required include X-ray machines and power supply systems for electrically heated catalysts (e.g., catalysts which are not continuously operated, such as in the case of catalytic converters used in automobiles). In each of the cases, the entire electric system (i.e., all control components), must be sized to accommodate the second high power state.
U.S. Pat. No. 5,041,776, assigned to Isuzu Motors, Ltd., discloses a power supply device for use on a motor vehicle for supplying stored electric power to an electric load such as a vehicle engine starter motor. This patent discloses using a parallel arrangement of a battery and capacitor. U.S. Pat. Nos. 5,155,373, 5,155,374 and 5,146,095, all assigned to Isuzu, disclose similar systems. For example, U.S. Pat. No. 5,155,374 allows higher voltage to be supplied to a vehicle engine starter motor by serially stacking a battery and a capacitor. The capacitor is recharged at a fixed rate using relatively complex circuitry. A serial connection of the battery and capacitor permits a doubling of the voltage supplied. By doubling the voltage via use of the capacitor, peak charge requirements of the battery can be reduced.
Although the Isuzu patents generally disclose using a capacitor in parallel with a battery, these patents do not disclose criteria for optimizing the relative size and power capabilities of the battery and the capacitor to reduce peak power requirements of the battery. Further, the circuits described in these patents merely permit the battery voltage to be doubled by, for example, serially connecting the battery and capacitor.
Accordingly, there is a need for identifying criteria which can be used to optimize relative size and power capabilities of a power source and a capacitor included in a power supply system so that energy management of the power supply system can be optimized. Further, there is a need for an electric system which can accommodate plural states, or operating conditions, without significantly limiting system design flexibility.